zum Directory-modus

Annulenes, Heterocyclic and Polycyclic Aromatic Compounds

Annulenes

Annulenes
Annulenes are monocyclic compounds containing alternating ring double bonds, such as benzene, but of different sizes. According to systematic nomenclature, benzene is a [6]annulene, while cyclobutadiene is [4]annulene, whereas cyclooctatetraene is an [8]annulene, for instance.
Fig.1
[4]Annulene.
Fig.2
[6]Annulene.
Fig.3
[8]Annulene.

Though [10]annulene possesses 4n+2 π electrons, it is not aromatic because the ring is not plane, due to bonding angle strains. Nevertheless, annulenes of large ring sizes may also be aromatic. [18]Annulene, for instance, is aromatic. Its carbon-carbon bond lengths are of nearly the same size and it posseses 4n+2 π electrons (with n = 4).

Fig.4
[18]Annulene.

Exercise

<Page 1 of 3